The nanotech tool kit includes a growing collection of devices for imaging, measuring, and manipulating nanoscale components. Often the devices themselves are nanoscale. Now, a double-walled carbon nanotube forms a device able to weigh a single atom of gold. Unlike other tools of comparable sensitivity, the new nanotube mass sensors are small enough to be a components of nanoscopic systems, and they do not have to ionize or destroy molecules to measure their masses. From the Lawrence Berkeley National Laboratory (credit ScienceDaily) “Golden Scales: Nanoscale Mass Sensor from Berkeley Can Be Used to Weigh Individual Atoms and Molecules“:

There’s a new “gold standard” in the sensitivity of weighing scales. Using the same technology with which they created the world’s first fully functional nanotube radio, researchers with Berkeley Lab and the University of California (UC) at Berkeley have fashioned a nanoelectromechanical system (NEMS) that can function as a scale sensitive enough to measure the mass of a single atom of gold.

Alex Zettl, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division (MSD) and UC Berkeley’s Physics Department, where he is the director of the Center of Integrated Nanomechanical Systems, led this research. Working with him were members of his research group, Kenneth Jensen and Kwanpyo Kim.

“For the past 15 years or so, the holy grail of NEMS has been to push them to a small enough size with high enough sensitivity so that they might resolve the mass of a single molecule or even single atom,” Zettl said. “This has been a challenge even at cryogenic temperatures where reduced thermal noise improves the sensitivity. We have achieved sub-single-atom resolution at room temperature!”

The new NEMS mass sensor consists of a single carbon nanotube that is double-walled to provide uniform electrical properties and increased rigidity. One tip of the carbon nanotube is free and the other tip is anchored to an electrode in close proximity to a counter-electrode. A DC voltage source, such as from a battery or a solar cell array, is connected to the electrodes. Applying a DC bias creates a negative electrical charge on the free tip of the nanotube. An additional radio frequency wave “tickles” the nanotube, causing it to vibrate at a characteristic “flexural” resonance frequency.

When an atom or molecule is deposited onto the carbon nanotube, the tube’s resonant frequency changes in proportion to the mass of the atom or molecule, much like the added mass of a diver changes the flexural resonance frequency of a diving board. Measuring this change in frequency reveals the mass of the impinging atom or molecule.

…While scientists already have the ability to measure the mass of individual atoms through a complex technique known as mass spectrometry, this new carbon nanotube NEMS mass sensor offers some distinct advantages and opens the door to new possibilities, as Jensen explained.

“Unlike mass spectrometry, our device does not require the ionization of neutral atoms or molecules that can destroy samples such as proteins. Also unlike mass spectrometers, our carbon nanotube mass sensor becomes more sensitive at higher mass ranges, which makes it more suitable for measuring large biomolecules like DNA. Finally, our device is small enough so that, in time, it could be incorporated onto a chip.”